The background description includes information that may be useful in understanding the present inventive subject matter. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed inventive subject matter, or that any publication specifically or implicitly referenced is prior art.
Friction stir welding (“FSW”) is a solid-state welding process in which a rotating tool heats and intermixes two workpieces at a seam. More specifically, the rotating tool has a pin that is pressed into the seam as the tool rotates, producing frictional heat between the tool and the workpieces. Enough heat is generated such that regions of the workpieces plasticize. A shoulder of the FSW tool assists in causing the plasticized regions to intermix, thus joining (i.e., friction stir welding) the workpieces at the seam. The rotating tool travels along the entire length of the seam to form a weld joint line between the two workpieces.
FSW provides numerous advantages over other welding processes, in part, due to the fact that FSW occurs at much lower temperatures and without a filler material. Some of the advantages of FSW include: better mechanical properties at the weld; less porosity, shrinkage, and distortion; little or no toxic fume emissions; no consumable filler material; and ease of automation. Since its conception in 1991, FSW has been heavily researched and successfully applied to numerous industries in a wide variety of applications.
Of particular importance in the present application is the welding of high strength workpieces such as high strength steel. European Patent Application Publication No. 1796865, for example, describes FSW processes for joining high strength steel pipelines. In such applications, the FSW tool must be made of a material that has exceptional strength, hardness, wear resistance, and has a high melting temperature. Such materials can be very expensive. Since FSW tools experience thermal wear, mechanical wear, and chemical wear, especially at the tip and shoulder, and eventually need to be replaced, the cost of FSW tools for FSW processes involving high strength workpieces can be prohibitive. It would be advantageous to provide a FSW tool that has a graduated compositional change, thus reducing the overall cost of the tool.
Various references describe coatings for FSW tools. The coatings improve the material properties of the tool tip for welding high strength materials. For example, International Patent Application Publication No. WO0185385 to Nelson describes a FSW tool that has a super abrasive coating (e.g., polycrystalline cubic boron nitride or polycrystalline diamond). Japanese Patent Application Publication No. JP2003326372A describes a diamond coated FSW tool.
Other references describe welding high strength materials with FSW tools that have been treated with surface hardening techniques. See, for example, Japanese Patent Application Publication No. JP2009255170 and International Patent Application Publication No. WO2009119543.
Coatings and surface hardening treatments can improve a FSW tool's hardness, wear resistance, and frictional/heat generation characteristics. While such FSW tools provide useful improvements for welding high strength materials, they fail to address the high costs associated with the actual base material of the FSW tools.
European Patent Application Publication No. EP1918059 to Park describes a FSW tool for welding of a wide range of materials, from aluminum to high strength steel. The FSW tool's pin is made of a Mo alloy that has a dual phase microstructure of Mo and intermetallic compound Mo5SiB2. Park further describes a FSW tool shank that is made of a different material than the pin. To some extent, Park addresses the high costs of FSW tools for high strength materials by providing a FSW tool that uses a different composition for the tool tip and the tool shank. However, Park fails to appreciate that the tool tip (see Park, FIG. 2, element 201) could have a graduated change of composition.
All materials and references cited herein are incorporated by reference to the same extent as if each individual reference were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Thus, there is still a need for improved FSW tools.